Work pieces such as strips or sheets consisting of an aluminum alloy are often surface treated after finishing rolling to prepare them for the next manufacturing step. In particular strips or sheet for lithographic printing are conditioned to achieve a predetermined surface roughness in a subsequent graining process. Litho-strips or sheets are usually degreased after finishing rolling. As known from the US-patent specification U.S. Pat. No. 5,997,721, degreasing respectively cleaning of the surface is done in one step by anodising the aluminum alloy sheet with AC current in an acidic electrolyte bath. Another way to degrease or clean aluminum slivers is known from the German patent DE 43 17 815 C1 namely the use of an alkaline medium. But from the use of alkaline media it is known that they do not remove every features of the subsurface microcrystalline layer, in particular oxide particles, which are present on or near the surface of the rolled aluminum strips.
However, prior electro-chemical graining the litho-strips are usually subjected to sodium hydroxide in a pre-treatment to degrease and clean the surface again, which process together with the electro-chemical graining is herein further called surface roughening process of litho-strips. In principle surface roughening is done by the manufacture of lithographic printing plates. Due to the increasing manufacturing speed of surface roughening of the litho-strips time for the pre-treatment of the surface of the litho-strips and for the electro-chemical graining decreases. It has been found that due to the increasing manufacturing speed the pre-treatment with sodium hydroxide is not sufficient enough to remove all contaminants from the surface of the litho-strip. As a consequence, the results in electro-chemical graining are not stable and surface defects occur on electro-chemically grained litho-strips or sheets. However, a reduction of the manufacturing speed causes higher production costs for lithographic printing plates.
Furthermore, methods of conditioning the surface of a litho-strip including two steps require relative high expenses related to facility equipments.